1. Field of the Invention
The present invention relates to elastic wave devices used as, for example, resonators and band-pass filters, and particularly, to an elastic wave device in which an IDT electrode having a plurality of electrode fingers has an improved structure.
2. Description of the Related Art
Elastic wave devices have been used widely as resonators and band-pass filters. Known examples of elastic wave devices are surface acoustic wave devices that utilize surface acoustic waves and boundary acoustic wave devices that utilize boundary acoustic waves.
Japanese Unexamined Patent Application Publication No. 2004-236285 discusses a boundary acoustic wave resonator in which first and second solid layers are stacked one on top of the other, and at least one normal IDT electrode (interdigital electrode) is disposed in the interface between the first solid layer and the second solid layer. A boundary acoustic wave device of this kind or a surface acoustic wave device is generally equipped with an IDT electrode having a plurality of electrode fingers in order to excite boundary acoustic waves or surface acoustic waves.
Japanese Unexamined Patent Application Publication No. 2004-260543 discloses a surface acoustic wave device having a weighted IDT electrode. FIG. 18 is a schematic plan view showing an electrode structure of the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2004-260543.
A surface acoustic wave device 101 is a filter device that utilizes surface acoustic waves and has input terminals IN and output terminals OUT. Moreover, first and second filters are provided on a piezoelectric substrate so as to constitute the electrode structure shown in the drawing. The first filter has a weighted excitation IDT electrode 102 and a normal-type receiving IDT electrode 103. On the other hand, the second filter has a normal-type excitation IDT electrode 104 and a weighted receiving IDT electrode 105. One of the weighted IDT electrodes 102, 105 is weighted so as to have the minimum phase characteristics, whereas the other is weighted such as to have the maximum phase characteristics.
The weighting is implemented by a crossing width weighting technique. For example, in the case of adjacent electrode fingers 102a and 102b in the IDT electrode 102, a gap 102c provided at the tip end of each electrode finger 102a and a gap 102d provided for each electrode finger 102b are provided at different positions in a direction perpendicular to the propagation direction of surface acoustic waves, or in other words, in the direction in which the electrode fingers 102a, 102b extend.
In the boundary acoustic wave device according to Japanese Unexamined Patent Application Publication No. 2004-236285, a normal IDT electrode is disposed in the interface between the first and second solid layers. In the surface acoustic wave device or boundary acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2004-260543, a technique in which the IDT electrodes are crossing width weighted to attain desired filter characteristics and resonance characteristics has been widely used.
However, in the boundary acoustic wave devices and surface acoustic wave devices of the related art, even with the use of weighted IDT electrodes, it is still difficult to attain sufficient resonance characteristics and filter characteristics. For this reason, improvements in the characteristics are in great demand.